Understanding JavaScript Truthy and Falsy


Can someone please explain JavaScript Truthy and Falsy, using the below sample data. I have read other threads but still confused.

var a = 0;

var a = 10 == 5;

var a = 1; 

var a = -1;

From my understanding, I believe that var a = 1; is the only truthy and the rest are falsy - is this correct?

From my understanding, I believe that var a = 1; is the only truthy and the rest are falsy’s - is this correct?

No.

  1. var a = 0;

    Number zero is falsy. However, note that the string zero "0" is truthy.

  2. var a = 10 == 5;

    This is same as var a = (10 == 5);, so this is falsy.

  3. var a = 1;

    var a = -1;

    Any non-zero number including negative numbers is truthy.

Quoting from MDN

In JavaScript, a truthy value is a value that translates to true when evaluated in a Boolean context. All values are truthy unless they are defined as falsy (i.e., except for false, 0, "", null, undefined, and NaN).

List of falsy values in JavaScript:From MDN

  1. false
  2. null
  3. undefined
  4. 0
  5. NaN
  6. '', "", ``(Empty template string)
  7. document.all
  8. 0n: BigInt

There's a simple way to check, which you can use now and forever:

function truthyOrFalsy(a) {
    return a ? "truthy" : "falsy";
}

To wit:

> truthyOrFalsy(0)
"falsy"
> truthyOrFalsy(10 == 5)
"falsy"
> truthyOrFalsy(1)
"truthy"
> truthyOrFalsy(-1)
"truthy"

Also see a list of all falsey values in JavaScript.


Truthy -> Value that resolve to true in boolean context

Falsy -> Value that resolve to false in boolean context


For better understanding, falsy values is given below.

  1. false
  2. 0
  3. empty string
  4. null
  5. undefined
  6. NaN

FALSY

  • false
  • 0 (zero)
  • "", '', `` (empty strings)
  • null
  • undefined
  • NaN (not a number)

note : Empty array ([]) is not falsy

TRUTHY

  • Everything that is not FALSY

In JavaScript, && and || don't always produce a boolean value. Both operators always return the value of one of their operand expressions. Using the double negation !! or the Boolean function, "truthy" and "falsy" values can be converted to proper booleans.

true && true =>
true

true && false =>
false

true && 'rahul626'=>
"rahul626"

true && 'i am testing Truthy' && ' upvote it'=>
" upvote it"

Try, tried on console

Well described here


In short there are only 6 types of falsy values: You can use this snippet to test them:

function isTruthy(val){
    if(val){
        console.log(val + ' is Truthy');
    }else{
        console.log(val + ' is falsy');
    }
}
    

// all below are truthy
isTruthy (true)
isTruthy ({})
isTruthy ([])
isTruthy (42)
isTruthy ("0")
isTruthy ("false")
isTruthy (new Date())
isTruthy (-42)
isTruthy (12n)
isTruthy (3.14)
isTruthy (-3.14)
isTruthy (Infinity)
isTruthy (-Infinity)

//all below are falsy
isTruthy(0);
isTruthy("");
isTruthy(false);
isTruthy(NaN);
isTruthy(null);
isTruthy(undefined);

Refer this site for details: https://developer.mozilla.org/en-US/docs/Glossary/Falsy


Easy way to check Falsy Value and True value

function truthyOrFalsy(val){
  if(val){
    console.log (`${val} is truthy`);
  } else{
    console.log (`${val} is falsy`);
  }   
}

Check all FALSY value:

truthyOrFalsy(false);      //Output: false is falsy
truthyOrFalsy(null);       //Output: null is falsy
truthyOrFalsy(0);          //Output: 0 is falsy
truthyOrFalsy('');         //Output:  is falsy  [blank refers to '']
truthyOrFalsy(NaN);        //Output: NaN is falsy
truthyOrFalsy(undefined);  //Output: undefined is falsy

Please note that undefined is not explicitly used to set as value. Some common scenarios will create undefined:

  • Parameter defined in function but not passed argument in callback function.
  • If nothing returns in function
  • If accessing to an object property/method which is not defined
  • If accessing to an array element which is not defined
function add(num1, num2){   
    console.log(num1, num2);    
}
const result = add(44);
console.log(result);
//Output: 44 undefined
//        undefined

const car = {color:"Blue", price: 200000};
console.log(car.category);
//Output: undefined
arrColors = ["Blue", "Sky", "Purple"];
console.log(arrColors[5]);
//Output: undefined

Check all TRUTHY values

All values are truthy unless they are defined as falsy.

Although ' ', '0', -1, [] could be enlisted to be checked.

truthyOrFalsy(' ');      //Output: is truty     [blank refers to space inside 
                         //                       quote ]
truthyOrFalsy('0');       //Output: 0 is truty 
truthyOrFalsy([]);          //Output: is truty  [blank refers to an empty array]
truthyOrFalsy(-1);         //Output: -1 is truty